[Antennas] Expanding foam and antenna tunning

2002年8月26日 13:44:52 -0400

Let me relate to you a case from my actual experience:
 I worked for the US Army Strategic Communications Command in the
50's and was tasked to come up with an antenna for 108 MHz at 50 kW that
would put all the energy in a pattern straight up in the air. After I
designed the antenna an had the prints drawn up it was sent to the US
Navy Yard, Washington, DC, to be built. Because the matching section
where the antenna (a dipole made of 3� inch copper tubing) connected to
the 3� inch Spyroflex coax was all designed for air dielectric and was
all open to the air, I specified that the antenna should be covered with
a "radome" as used on many radars of that day.
 After the Navy Yard had the specs and drawings for a few weeks I got
a call from the man in charge of constructing the antenna stating that
instead of a radome covering the entire antenna, he was going to use a
polyfoam to seal up the part that I had designed for air dielectric. I
told him I didn't think that was a viable alternative, because of the
difference in dielectric constant between all-air and the foam stuff,
and I wrote him a memo for record so stating.
 The Navy Yard went ahead and used the foam filling against my
opinion. And the antenna was sent to NASA, the intended user.
 I heard later, through the government "grape vine" when the antenna
was fired up it lasted about 3 minutes and burst into flames!!
 Conclusion: don't use the foam filler without redesigning the
antenna to take into consideration the difference in dielectric constant
between air and the foam and considering the loss that may be introduced
in the form of heat in the foam dielectric.
Harvey/W4TG